Tell Doxygen to generate XML

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/doxygen/mbedtls.doxyfile b/doxygen/mbedtls.doxyfile
index c33c7e3..9dca927 100644
--- a/doxygen/mbedtls.doxyfile
+++ b/doxygen/mbedtls.doxyfile
@@ -42,3 +42,5 @@
 #     \retval #PSA_ERROR_INSUFFICIENT_MEMORY \emptydescription
 # This avoids writing redundant text and keeps Clang happy.
 ALIASES += emptydescription=""
+
+GENERATE_XML           = YES